The present invention relates to position computation methods and systems, and more particularly, to methods and systems for obtaining assistance information from a communication network for use in position computation. Wireless communication systems (networks) are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a conventional terrestrial wireless communication system 20 that may implement any one of the aforementioned wireless communications standards. The wireless system may include one or more wireless mobile terminals (stations) 22 that communicate with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO 28 and may serve thousands of wireless mobile terminals 22.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between wireless mobile stations (terminals) 22 and a MTSO 28, by way of the base stations 26 servicing the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the communication system 20, a duplex radio communication link 30 may be effected between two wireless mobile stations 22 or between a wireless mobile station 22 and a landline telephone user 32 via a public switched telephone network (PSTN) 34. The function of the base station 26 is commonly to handle the radio communications between the cell 24 and the wireless mobile terminal 22. In this capacity, the base station 26 functions chiefly as a relay station for data and voice signals.
As the wireless communication industry continues to advance, other technologies will most likely be integrated within these communication systems in order to provide value-added services. One such technology being considered is a global positioning system (GPS). Briefly, as illustrated in FIG. 2, GPS is a space-based triangulation system using satellites 52 and computers 58 to measure positions anywhere on the earth. GPS was first developed by the United States Department of Defense as a navigational system. The advantages of this navigational system over other land-based systems are that it is not limited in its coverage, it provides continuous 24-hour coverage, regardless of weather conditions, and is highly accurate. While the GPS technology that provides the greatest level of accuracy has been retained by the government for military use, a less accurate service has been made available for civilian use. In operation, a constellation of 24 satellites 52 orbiting the earth continually emit a GPS radio signal 54. A GPS receiver 56, e.g., a hand-held radio receiver with a GPS processor, receives the radio signals from the closest satellites and measures the time that the radio signal takes to travel from the GPS satellites to the GPS receiver antenna. By multiplying the travel time by the speed of light, the GPS receiver can calculate a range for each satellite in view. Ephemeris information provided in the satellite radio signal typically describes the satellite's orbit and velocity, thereby generally enabling the GPS processor to calculate the position of the GPS receiver 56 through a process of triangulation.
The startup of a GPS receiver typically requires the acquisition of a set of navigational parameters from the navigational data signals of four or more GPS satellites. This process of initializing a GPS receiver may often take several minutes.
The duration of the GPS positioning process is directly dependent upon how much information a GPS receiver has initially. Most GPS receivers are programmed with almanac data, which coarsely describes the expected satellite positions for up to one year ahead. However, if the GPS receiver does not have some knowledge of its own approximate location, then the GPS receiver cannot find or acquire signals from the visible satellites quickly enough, and, therefore, cannot calculate its position quickly. Furthermore, it should be noted that a higher signal strength is needed for capturing the C/A Code and the navigation data at start-up than is needed for continued monitoring of an already-acquired signal. It should also be noted that the process of monitoring the GPS signal is significantly affected by environmental factors. Thus, a GPS signal which may be easily acquired in the open typically becomes harder to acquire when a receiver is under foliage, in a vehicle, or worst of all, in a building.
Recent governmental mandates, e.g., the response time requirements of the FCC Phase II E-9 11 service, make it imperative that the position of a mobile handset be determined accurately and in an expedited manner. Thus, in order to implement a GPS receiver effectively within a mobile terminal while also meeting the demands for fast and accurate positioning, it has become desirable to be able to quickly provide mobile stations with accurate assistance data, e.g., local time and position estimates, satellite ephemeris and clock information, and visible satellite list (which generally varies with the location of the mobile station). The use of such assistance data can permit a GPS receiver that is integrated with or connected to a mobile station to expedite the completion of its start-up procedures. It is, therefore, desirable to be able to send the necessary GPS assistance information over an existing wireless network to a GPS receiver that is integrated with or connected to a mobile station.
Taylor et al., U.S. Pat. No. 4,445,118, discusses the concept of aiding or assisting GPS receivers. The method described uses a single transmitter, such as a geosynchronous satellite, to provide a single assistance message for a wide geographical area. The assistance data includes a list of GPS satellites in view, the respective satellite positions, and predicted Doppler shifts on the satellite signals. This structure of this message permits the position computation function (PCF) to be done in the user receiver.
Krasner, U.S. Pat. No. 5,663,734, describes another GPS receiver approach. The patent is mainly related to the receiver architecture, but discusses how the receiver performance can be improved by assistance. The patent mentions "data representative of ephemeris" and expected Doppler shifts as possible contents of the assistance message.
Lau, U.S. Pat. No. 5,418,538, describes a system and method for aiding a remote GPS/GLONASS receiver by broadcasting "differential" information from a like receiver in a "reference station". The reference station broadcasts a visible satellite list and also the associated ephemeris, in one embodiment. The advantage to the remote receiver is three-fold: reduced memory requirements, lower-cost frequency reference, and faster acquisition. The discussion describes the benefit of being able to estimate and remove the Doppler due to the receiver clock inaccuracy after acquiring the first satellite.
Eshenbach, U.S. Pat. No. 5,663,735, describes a method whereby a GPS receiver derives an accurate absolute time reference from a radio signal. Optionally, the receiver also derives from the radio signal a frequency reference that is more accurate than the inexpensive crystal oscillator contained in the receiver. The GPS receiver performs the position calculation, and therefore must have the absolute time as well as the ephemeris and clock corrections for the GPS satellites.
Another assisted-GPS proposal for GSM-based networks is T1 standards documents T1P1/98-132r2. This proposal is based on placing reference GPS receivers at various nodes in the network, capturing the ephemeris information from these receivers, then providing this information along with a list of visible satellites to all handset-based GPS receivers via messages on GSM downlink bearers. The benefit of this approach is that it allows the handset-based GPS receiver to be fully functional, i.e., it contains the PCF and also can operate in continuous navigation mode.
One particularly challenging but important component for which assistance would be beneficial is obtaining accurate GPS timing information at the GPS receiver. Traditionally, GPS receivers demodulate the required timing information from the messages broadcast by the GPS satellites. However, reasonably error--free demodulation of such signals may not be possible below a certain signal threshold which itself may be significantly higher than the minimum signal level required for tracking already acquired signals and making range measurements. Accordingly, where GPS receiver operation is desirable under conditions of low-signal operation (for example, due to environmental attenuation, antenna compromises or other affects) it may not be possible to rely on demodulation of the transmitted information from the GPS satellites as a source of GPS timing information.
As noted above, one alternative approach previously proposed is the use of assistance information from a cellular network which may be provided to the combined GPS and cellular receiver by the serving cellular network in some manner. Three different approaches for providing such GPS timing information through network assistance have previously been proposed. First, some networks are synchronized by GPS. An example is the IS-95 CDMA system which, as a result, has an implicit timing relationship between the air-interface timing (i.e. the spreading codes) of the communication network and GPS timing. Therefore, once a GPS-equipped mobile station (GPS-MS) synchronizes with the communication network air-interface it is expected to also have accurate GPS timing that can be used to improve the sensitivity and time-to-first-fix (TTFF) of the GPS receiver in the device. This approach is only useful, however, for communication networks, such as IS-95 CDMA, which have such an implicit timing relationship.
One approach proposed for networks that are not so GPS-synchronized is to establish a relationship between GPS timing and a communication network's air-interface timing at each cell transmitter (base station) of the communication network by provision of an observer unit equipped with a GPS receiver as well as a cellular receiver. This timing relationship information can then be reported to a GPS assistance server of the communication network and thereby included in assistance messages sent to GPS-MS devices being serviced by the respective base station of the communication network. Accordingly, once a GPS-MS device synchronizes with the air-interface timing of its serving cell of the communication network and receives this timing assistance, it may determine the current GPS timing accurately. Systems incorporating this second approach are described in U.S. patent application Ser. No. 09/264,120 filed Mar. 8, 1999 and entitled Method and System for Aiding GPS Receivers Via a Cellular or PCS Network and in U.S. patent application Ser. No. 09/219,199 filed Dec. 22, 1998 and entitled System and Method For Cold Start of a GPS Receiver in a Telecommunications Environment, both of which are hereby incorporated by reference herein as if set forth in their entirety.
A third approach that may be applied to unsynchronized networks without a GPS observer unit (also referred to as a Location Measurement Unit (LMU)) at each base station location is described in U.S. Pat. No. 5,812,087 entitled Method and Apparatus for Satellite Positioning System Based Time Measurement. In this approach, the timing information is derived from samples of the navigation signal from multiple GPS satellites. For example, GPS-MS device may make measurements on the ranging codes of multiple GPS satellite signals and also sample some duration of the navigation data that is imposed on these codes. This data may then be returned to a server where the navigation data samples may be matched to the samples of a reference signal to estimate the time at which the other measurements were made.
Synchronization of timing through the network may not be feasible for a variety of reasons with these various approaches. For example, adding GPS receivers to each base station may be unduly costly and, in some regions of the world, there may even be political resistance to this approach given that the GPS system is operated by the United States Government. Furthermore, it may not be cost effective or feasible to retrofit existing communication networks with GPS synchronization capability. While the LMU approach may reduce the impact on existing communication networks to some degree, it is still subject to cost problems. The approach described in U.S. Pat. No. 5,812,087 may be burdened by the amount of data that must be returned to the server from the GPS-MS for each position computation. This may create excessive communication demands on a communication network for repetitive positioning types of applications such as navigation. Furthermore, this approach may not enable the GPS-MS to determine the time which is typically necessary to allow the positioning computation to occur locally at the GPS-MS as opposed to being performed at the server with the position computation result then being provided to the GPS-MS.
Therefore, it would be desirable to have lower cost ways to implement a wireless mobile terminal with a GPS receiver integrated therein while still obtaining benefits from the combined capabilities of the resulting device.